The present invention relates to correction of registration errors caused by chromatic aberration in an optical device (zoom lens, dichroic prism) of a multi-tube type TV camera.
FIG. 1 shows the constitution of a three-tube type color TV camera.
As is shown in FIG. 1, in a three-tube type color TV camera, a subject 1 is picked up through a zoom lens 2, and the optical image obtained is separated into tri-color images of red (R), green (G) and blue (B) by a dichroic prism 3, which are then respectively converted into electrical signals by individual camera tubes 4r, 4g and 4b. The numerals 5r, 5g and 5b indicate deflecting coils respectively attached to the camera tubes, and numeral 6 indicates a deflection circuit for supplying deflection currents to the deflection coils 5r, 5g and 5b. The primary color signals thereby obtained are fed to the display equipment through signal processing circuit 7r, 7g and 7b. At the display equipment, these three primary color signals are added to each other so as to form a reproduced picture 8.
The process of adding primary signals from a camera is called registration. Registration deviations (registration errors) are caused by the following factors.
(1) chromatic aberration caused in an optical device such as a zoom lens and a dichroic prism,
(2) variations in the geometric distortion characteristics of a camera tube and of a deflecting coil.
For the purpose of correcting a registration error caused by the second of these factors (2), one conventional method has been provided, in which a correction waveform such as a saw-tooth waveform and a parabolic waveform is added to a deflection waveform of the deflection circuit 6 shown in FIG. 1 so as to alter the position of an electron beam on the scanning area of the camera tube, thus correcting the error. Recently, another method known as the digital registration correcting method (disclosed in Japanese Patent Application Kokai No. 57-2166) has enabled highly accurate correction.
While registration errors caused by the first factor (1) mentioned above have been the subject of some studies, they have not been the object of correction in the method previously proposed or practiced, because this kind of distortion is one which changes dynamically while the other factors consist of static distortions.
Because of the recent progress made in the development of TV cameras such as the development of a high definition TV camera, the correction of registration errors caused by the problem (1) of chromatic aberration in optical devices has become a matter requiring urgent attention so that the overall characteristics of TV cameras may be improved.
FIG. 2 shows the general characteristic of a chromatic aberration in an optical device. The axis of ordinate in this figure represents a distance "h" between the aberration and the optical axis 0 (an image height) on the focal plane when picked up through a lens, and the axis of abscissa represents the amount of distance, namely the degree of aberration E (h), of the red or blue image from the green image.
Generally, a form of chromatic aberration becomes radial outwardly from the center which is the optical axis, and the degree of aberration is even at a coaxial circle around the optical axis.
This chromatic aberration varies, as indicated by the characteristic curves 9-12 in FIG. 2, in accordance with changes of the aperture stop or iris number, the zoom ratio and the focal length.
If the characteristic of the chromatic aberration curves to the side of (+) in accordance with increase of the distance "h" from the optical axis, as indicated by the characteristic curve 9 in FIG. 2, the registration error thereby caused takes the form of what is called a pincushion, as shown in FIG. 3B. Conversely, if the characteristic curves to the side of (-) as indicated by the characteristic curves 11 and 12 in FIG. 2, the registration error takes the form of a barrel, as shown in FIG. 3C. If the chromatic aberration has a linear characteristic as indicated by the characteristic curve 10 in FIG. 2, the registration error is such as is shown in FIG. 3A in which the angle of field is enlarged (or reduced) at a constant ratio in every respect.
According to one conventional method for correcting chromatic aberration, as shown in FIG. 4, a characteristic curve 13 of a chromatic aberration is approximated by a straight line such as a broken line 14 so as to make a correction waveform corresponding to this straight line, and this correction waveform is added to an electron beam deflection waveform of a deflection circuit, thus correcting the chromatic aberration (this method is disclosed in Japanese Patent Application Kokai No. 49-87237). In this case, the amount of remaining errors is such as indicated by a alternate long and short dash line 15 in FIG. 4.
The above mentioned ensures the correction of high accuracy when the characteristic of the aberration is substantially linear. However, when the curavature is as large as the characteristic curve 13 shown in FIG. 4, a large distortion remains after the correction is carried out, as illustrated by the alternate long and short dash line 15 shown in FIG. 4. This results not only in the correction with inadequate accuracy and a large distortion, but also in a large amount of registration errors at a point having a small distance "h" from the optical axis, namely at the central portion of the image, than the state before the correction is made, as is apparent from comparison between the characteristic curve 13 and the alternate long and short dash line 15 shown in FIG. 4. Generally, the central portion of the image is given the highest resolution through the camera system (lenses, camera tubes) and is important in terms of visual sensation. Accordingly, the elimination of the registration errors is an important problem.